Study Of The Hydraulic Characteristics Of Clearwells And Its Applications

This research, affiliated to the "Designing Principles for Novel Clearwell andits Application", a sub-project of a state-level key research project (863 Project)"Technologies for Safe Drinking Water Supply in South China", was conducted toinvestigate the factors that influence the hydraulic characteristics and effectivehydraulic residence time at both pilot level and full-scale level clearwells withdifferent configurations, using the tracer testing and Computational Fluid Dynamics(CFD) method.In this dissertation, the concept of effective hydraulic residence time (T10) wasprovided for evaluating the hydraulic efficiency of clearwells. T10 represented thetrue residence time in clearwells of disinfectants. The clearwells with differentconfigurations were found to have different effective hydraulic residence time eventhough they had same theoretical hydraulic residence time (T). The hydrauliccharacteristics and effective hydraulic residence time (T10) could be obtained eitherby tracer testing or Computational Fluid Dynamics (CFD) method. With versatilefunction of CFD method, the flow fields and other hydraulic phenomena insideclearwells were visualized. Thus, with the additional benefits of saving testing timeand cost, the CFD method can be applied to optimize clearwell designing andretrofiting.The results of tracer test on pilot level, which was 1/38 scale of full-scaleprototype shown that the ratio of effective hydraulic residence time and theoryhydraulic residence time (T10/T) was logarithmically related with the ratio of lengthto the width (L/W) of the water channel inside clearwells and the relationship couldbe expressed as (T10/T) = 0.1894Ln(L/W) -0.0494. It was found that T90/T and σ t2of clearwells had the power relation with L/W. The values of T90/T and σ t2decreased with the increase of L/W. The tracer test of four full-scale clearwells hadalmost same result as the pilot test. It was also observed that when the L/W exceededsome value, the increase trend of T10/T slowed down.The feasibility of CFD method was verified by the results of pilot testing. It wasfound that inserting perforated plates, enlarging the inlet and outlet diameters andusing arc-shape turning could also improve the hydraulic efficiencies of clearwells.Through the retrofitting of clearwells, it was expected that maximizing disinfectionand minimizing DBP formation could be achieved simultaneously.CFD method also was used for designing the demo full-scale clearwellsaffiliated to the 863 project, which now are under construction. It was found that theT10/T of two clearwells would be improved with the setup of more baffles. Theprimary simulations of chlorine diffusion in the two clearwells were also conductedand the distribution of chlorine was improved in modified clearwells.